Study of Orangutans Yields New Ideas about Human Evolution

Results from research conducted by a team of scholars and scientists on the dietary lives of orangutans in tropical Borneo have given possible clues to how very early human ancestors may have adapted, survived and changed millions of years ago. In addition, the results may help scientists better understand eating disorders and obesity in human populations today.

Led by evolutionary anthropologist Erin Vogel of Rutgers University, the research team analyzed samples of compounds and byproducts in Orangutan urine over a 5-year period to determine the effects of protein recycling in their dietary, or eating behavior. What they found was that they have been able to survive through prolonged protein deficits by eating higher protein leaves and the inner bark of trees during lean times, as well as burning the energy from stored body fat and eventually muscles for extended times when their preferred diet of fruit is not available.

Borneo presents a very challenging environment for some of its faunal inhabitants. Many of the food-bearing plants they rely upon only produce appreciable quantities of fruit every four or five years, and when they do bear, the entire forest environment produces all at once. Animals, and particularly the orangutans, living there are thus forced to gorge themselves and gain a large amount of fat, living off their body fat reserves for the following three to four years during the cyclical "starving time". During the lean years they turn to hard foods, such as hard seeds and the starchy tissues beneath the bark of trees, supplementing the energy derived from the burning of their body fat and thus surviving an environment that otherwise would lead to starvation and death. Moreover, uncontrolled deforestation by modern humans have also reduced the habitat availability, making an already challenging situation even more difficult, but apparently thus far survivable.

Anthropologists have long theorized that early human ancestors, or hominins, had to adapt to similar circumstances as their environment changed. Says Nathaniel Dominy (pictured below), associate professor of Dartmouth College and a key member of the research team, "we are interested in how orangutans cope with food-limited environments because it may give us a glimpse into what early human ancestors were facing".

Why relate or compare the orangutans of Borneo to the early hominins? It is because anthropologists suggest that the apes that preceded and gave rise to the earliest human ancestors sported teeth much like those of orangutans. The likeness is especially acute between the teeth of pre-human apes and those of the orangutans of Borneo. Dominy suggests that these orangutans' diets may have acted as a selective pressure to develop their molar teeth, explaining a possible natural selection paradigm for early human-ancestral apes. As the larger molars and more robust jaws of the Borneo orangutans are theorized to have developed in response to the hard, tough diet during the lean years, so suggests the researchers that early hominins, who's large molar teeth have been observed to exhibit similar wear patterns, were the product of similar dietary challenges.

"Perhaps the hard objects were things they ate only very occasionally under ecological duress," Dominy adds. "It is not what they ate regularly that matters. It is what they were eating during crunch times. Because they routinely go through these dire times, orangutans may be a good model for what happened to human ancestors in deep time."

The Implications for Human Diets Today

Team leader Erin Vogel further suggests that the study may also help scientists obtain a better understanding of human obesity and eating disorders today, a problem that has beset the human condition of modern human populations.

Along these lines, the research indicated that it was during periods of high calorie and protein intake that orangutans put on weight and fat, and that they dip into these fat reserves and even into their protein (muscle) reserves to survive during the lean times, resulting in weight loss. This is not altogether surprising, but it is a fact that does not fully support the belief by some that a high protein, low carbohydrate diet is the best way to lose weight.

"There is such a large obesity epidemic today and yet we don't really understand the basis of the obesity condition or how these high-protein or low-protein diets work. I think studying the diets of some of our closest living relatives, the great apes; may help us understand issues with our own modern day diets," she said.

Details of the study can be found in the December 14, 2011 online issue of Biology Letters, a journal of The Royal Society.